An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non planar surface. In addition, planarization of the substrate surface is usually required for photolithography.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing liquid, such as a slurry with abrasive particles, is supplied to the surface of the polishing pad.
In order to determine the effectiveness of a polishing operation, a “blank” substrate (e.g., a wafer with multiple layers but no pattern) or a test substrate (e.g., a wafer with the pattern to be used for device wafers) is polished in a tool/process qualification step. After polishing, the substrate is removed from the polishing system and the remaining layer thickness (or another substrate property relevant to circuit operation, such as conductivity) is measured at several points on the substrate surface using an in-line or stand-alone metrology station. The variation in layer thickness provide a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. The in-line or stand-alone metrology station can provide extremely accurate and reliable thickness measurements (e.g., using ellipsometry) and precise positioning of a sensor to desired measurement locations on the substrate. However, this metrology process can be time-consuming, and the metrology equipment can be costly.
One problem in CMP is determining whether the polishing process is complete (i.e., whether a substrate layer has been planarized to a desired flatness or thickness). Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, for some applications, determining the polishing endpoint merely as a function of polishing time can lead to unacceptable variations in the post-polishing thickness of the substrate layer. However, removal of the substrate from the polishing apparatus for transportation to an in-line or stand-alone metrology station can lead to an unacceptable reduction in throughput.
Several methods have been developed for in-situ polishing endpoint detection. One class of methods involve optically monitoring the substrate during polishing, e.g., using an optical sensor positioned in the platen that directs a light beam through a window onto the substrate. However, measurements using such an in-situ system usually cannot be precisely positioned at a desired measurement location due to the motion of the substrate relative to the sensor, and the measurements can be less accurate due to noise generated by the polishing environment (e.g., absorption of light by slurry), the limited time available for measurements, and the need for real-time processing of the sensor data.